Lasers are used in many different industries for many different purposes, such as, for example, in the medical industry for medical procedures, in the printing industry in laser printers, in the defense industry in a variety of defense applications, and in the optical communications industry for transmitting and receiving optical signals. In many applications, the output power of the laser is monitored and controlled to maintain the output power at a desired or required level. Due to the wide variations in laser parameters such as, for example, laser slope efficiency (SE) and laser threshold current (ITH), maintaining the optical power at a particular level is challenging because temperature and process variations and aging of system components also cause the output power level to vary. Many techniques and systems have been used or proposed to control and maintain the output power of the laser at required levels over temperature and process variations and time.
It is common practice in the optical communications industry to use a monitor photodiode to detect light output from a rear portion of the transmitter laser (or a portion of the output power reflected back through optical lenses) and to use this optical feedback to measure and control the average transmitted output power level of laser. In general, the average transmitted output power level, PAVG, of the laser can be controlled by controlling the bias current, IBIAS, of the laser. Thus, if the optical feedback indicates that PAVG has fallen below the required level, increasing IBIAS by an appropriate amount will raise PAVG to the required level. Similarly, if the optical feedback indicates that PAVG has risen above the required level, decreasing IBIAS by an appropriate amount will lower PAVG to the required level.
As the optical feedback path described above is used to maintain PAVG at the required level, the laser is modulated with a modulation current, IMOD, to cause the laser output power level to be adjusted between a level, P1, that represents a logic 1, and a power level, P0, that represents a logic 0. The amplitude of the modulation current IMOD1 corresponds to an output power level of P1. The amplitude of the modulation current IMOD0 corresponds to an output power level of P0. The laser threshold current ITH has an amplitude that is sufficient to cause the laser to begin producing laser action (i.e., to emit stimulated radiation). The amplitude of the threshold current ITH needed to produce laser action varies due to factors such as, for example, temperature and aging. Due to these variations in the amplitude of ITH that is needed to produce lasing and the slope efficiency of the laser, adjustments must be made to the amplitudes corresponding to IMOD1 and IMOD0 in order to maintain P0 and P1 at the necessary respective output power levels.
A variety of techniques have been used to control the amplitude of the modulation current. One known technique sets the amplitude of the modulation current at a level that achieves a desired extinction ratio (ER) or optical modulation amplitude (OMA) at a fixed temperature or time. The amplitude of the modulation current is then increased or decreased based on an analog temperature coefficient, or in a digital control system, based on a temperature measurement and/or and aging timer. This technique generally provides suitable results if the laser SE variation is controlled well enough to maintain the ER/OMA and the laser performance within specifications. A disadvantage of this technique is that using a single temperature reference point means the adjustment to the amplitude of the modulation current is essentially based on a “guess” of changes of the SE in direction and amount based on statistical or other data. Because the change in the SE often is not linear and can change from positive to negative slope from one temperature to the next, it is difficult or impossible to determine the optimal adjustment in the amplitude of the modulation current. In addition, this technique can also limit laser yields because the SE and ITH limits need to be within sufficiently tight tolerances to guarantee that a suitable level of performance will be achieved without the necessity of testing each laser over temperature and customizing each laser based on the results of testing.
Another known technique involves measuring the amplitudes of the modulation current needed to maintain the required output power levels PAVG, P0 and P1 over a range of temperatures on a part-by-part or wafer-by-wafer basis. The amplitude values obtained during testing are programmed into a lookup table (LUT) memory element or other non-volatile memory element. A controller uses a temperature measurement value to index into the memory element and read out the corresponding amplitude value for the modulation current. The amplitude of the modulation current of the laser is then set to the value read out of the memory element. One disadvantage of this technique is that it requires over-temperature testing during manufacturing, which is expensive. In addition, it is difficult to factor in aging when using this technique, which means that the amplitude of the modulation current often will not be set to an optimal level.
Another technique that has been proposed involves using a high-speed monitor diode in combination with an amplitude detector to monitor the output power level of the laser and adjust the amplitude of the modulation current to achieve the required output power level. This technique requires an additional high-bandwidth feedback path for the amplitude detector, which increases the cost and complexity of the transmitter. In addition, the performance of the amplitude detector can be significantly affected by temperature variations, which can lead to less than optimal performance. Also, the amplitude detector dissipates a large amount of power relative to the rest of the transmitter. Because of these difficulties, this technique has been proposed, but not actually implemented.
Another known technique involves measuring ITH in situ by adjusting the modulation current amplitude while measuring the optical feedback signal to obtain the slope and calculate the corresponding SE. The measured ITH and SE are then used to calculate the amount of modulation current needed. An advantage of this technique is that the existing feedback path for the monitor photodiode is used. A disadvantage of this technique is that it requires a large amount of signal processing to be performed to make the necessary calculations, and is therefore computationally intensive. Another disadvantage is that the method is performed during module power up or module programming, and generally cannot be used while transmitting actual data.
A similar technique also uses the existing feedback path to calculate SE, but also modulates a very small amplitude signal at low frequency on top of the IBIAS supplied to the laser. This signal is then extracted from the feedback signal, amplified and used to calculate SE. The calculated SE is then used to determine how to adjust the modulation current amplitude. This method can be used while transmitting actual data, but requires that high accuracy circuits such as amplifiers, analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) be included in the feedback path. In addition, this technique also requires a significant amount of signal processing, and is therefore computationally intensive.
A need exists for a way to determine the adjustment needed, if any, to the amplitude of the modulation current in order to achieve a desired or required output power level, which can be used when transmitting actual data, which uses the existing feedback path, which does not require complex high-accuracy circuitry, and which does not require that over-temperature testing be performed.